I have a betta fish that I rescued from HORRIBLE circumstances about 4 months ago. (In one of those typical petshop betta cups, though the algae was so thick and green on the plastic that we were honestly not sure if there was a fish inside, let alone what color/size or type he was!)
When we let him out we saw that he actually looked healthy. No visible parasites, no signs of rot (lovely long fins), appetite, etc.
But about two weeks later his fins disappeared and never came back. He looks like a snake! Absolutely no fins on top
(totally flat back) stub of a tail, and one small string left as his underside fin. Its AMAZING he can swim/balance. He's looked like this for about two months. Swims actively, fantastic appetite, growing, etc. But has no fins! He lives alone in a 2.5 gallon tank with weekly water change. I've had Bettas my whole life and have never seen this.
Instinctively I'd address this as fin rot... but he's so healthy otherwise, and no treatments have brought back his fins. He shows none of the signs of the white "trim" of fin rot, he simply has none. What's going on??